Basic purposes and principles

There are many different types of brain implant used today; the simplest is a passive stimulator, like those used in deep brain and vagus nerve stimulation, which simply applies pulses to a group of neurons... more sophisticated stimulators can affect many distinct neurons, as in cochlear implants. Highly refined arrays of micro-electrodes allow transduction of useful sensory data into brain regions that can receive and interpret them. Arrays of electrodes can also be made to monitor neural activity, providing electrical outputs to the neural system.

As increasingly sophisticated electrical devices are integrated with neural tissues, an effective interface becomes highly significant. Where arrays are interacting with hundreds or thousands of neurons there must be a logical codec by which information can be exchanged meaningfully. Devices that receive and integrate inputs from the body often require tuning to an individual's specifications, as well as to the type of information being exchanged. The study of these neural correlates is termed neuro-informatics.

As is obvious from the language being used in this section, the concept of data or information is merging with ideas of functional brain activity. It makes sense that the wealth of knowledge on handling and manipulating data, information in all its forms, would provide useful handles for getting to grips with the flow of changes that is brain activity. While useful, computational models of brain function are not perfect, and should be seen as a stepping stone to deeper understanding.
[Andersen et al,2004]

Cochlear implants

People who have suffered loss of the hairs that normally detect vibrations in the ear can benefit by bypassing the hairs entirely and stimulating the neurons directly. A device using around 22 electrodes is implanted into the cochlea, which receives a radio signal from a nearby processing unit. This unit picks up sound though a microphone and analyses it before transmitting to the implant. By using band-pass filters the sound is split into frequency bands, similar to the ears natural way of differentiating frequency. Modern implants also contain processors that emphasise crucial vowel and consonant sounds to improve the users reception. Effective use requires substantial tuning, where an audiologist adjusts the device to suit the users needs. See a picture
[Litovsky et al,2004]

There is some controversy over the use of cochlear implants, especially in young children, where parents are encouraged to pay for the expensive operation to 'cure' their deaf child. Opponents are often deaf themselves or closely associated with deaf people. They describe such treatment as a brutal form of clinical eugenics, which is totally unnecessary, as shown by the ability of deaf people to live happy and free lives without any implants. Such debates are likely to continue, and even escalate as more kinds of implant become available.
[Hazell, 1994]

Sensory substitution

Pioneered for people who aren't able to experience a sense, where information is translated so as to be accessible by one of the functioning senses. One example is of a tactile-visual substitution where a camera translates optical input into pressure, sent to an array of solenoids that are placed on the body. Similar systems exist that translate images into sound, scanning from side to side, where volume is brightness and pitch is position vertically. Naturally a direct interface with the brain would allow greater throughput and adaptability. The key to such implants working is plasticity in the brain, as it must adapt to use the new incoming stimuli. Thus the young are most able to such devices, although perhaps the expanding knowledge of neurotrophins will make them accessible to elders too.
[Hanneton et al,1999]

Deep brain stimulation

An increasingly common treatment for movement disorders, being applied to other realms in recent years. See the DBS page for a detailed discussion.

Retinal implants

It is possible to create visual artifacts in humans using stimulation of the retina or visual cortex, using electrodes or trans-cranial magnetic stimulation. These artifacts are usually just flashes, but recently steps have been made to provide useful input to these areas.
Pioneering work has used tiny implants to stimulate the retina in partially sighted people. The implant consists of around 5000 tiny photo-diodes that convert light into electricity, just as retinal rods and cones do. Whole artificial retinas are also being tested, and results appear positive. Future developments may allow external modification of the signals produced by the implant, allowing vision of 'virtual light' - images formed by external cameras or computers could be viewed without a screen.
[Chow et al,2004]